1. Field of the Invention
The present invention relates generally to motor current feedback detection, and relates more particularly to reconstructing motor currents from DC bus currents.
2. Description of Related Art
Motor drives that switch DC power to control single or multiple phase motors are well known. A typical application involves switching DC bus current or power to different phases of a three phase AC motor. In controlling the AC motor, it is desirable to accurately measure motor phase current over a wide range of operating parameters.
It is known to measure motor phase current through current transformers or Hall effect sensors that are directly coupled to the motor phase lines that carry the current between the switches and the motor. However, these current sensors are typically large and costly, often making up a large percentage of the overall cost of the motor drive. In addition, the sensors are susceptible to non-linear operation and variations over time and with changing environmental properties, such as temperature. It would be desirable to obtain a measure of the motor phase currents without having to measure the current of each phase individually.
A popular control technique for controlling switching of the motor drive involves the use of space vector modulation. In space vector modulation, a rotating vector represents the motor shaft angle. The rotating vector is broken down into component vectors that represent individual switching states for controlling current to the motor. In this type of motor control, it is possible to measure motor phase current by measuring the DC bus current when non-zero basic vectors are used in the space vector modulation. FIG. 1 shows a pulse width modulated (PWM) inverter drive system, in which each basic vector is assigned a specific time in a PWM cycle to generate a command voltage vector. A space vector diagram is illustrated in FIG. 2, showing the various switching states and quadrants for space vector control. Each switching state represents ON and OFF switch conditions for each pole of the inverter A, B and C. For example, FIG. 3 illustrates the switch conditions for state [100] according to the vector VI of the space vector diagram.
Referring now to FIG. 4, the shaded areas of the space vector diagram illustrate non-observable regions near the sector borders, with the corresponding PWM waveforms. FIG. 5 illustrates non-observable regions in the case of a low modulation index, with the corresponding PWM signals. There are two zero vectors not illustrated in the above diagrams, which can be viewed in the abstract as coming out of the center of the vector diagram or going into the page, as illustrated in FIG. 2. With a PWM induction motor drive, the six active inverter states have a DC link current that is directly related to the current in one of the motor lines. In the zero vector state, the DC link current disappears because all motor currents are free wheeling within the inverter. The voltage vector and related DC link currents are illustrated in FIG. 6.
When switching between sector boundaries, it is difficult to determine the motor phase current because of the brief interval of the switching state, as illustrated by the shaded areas in FIG. 4. The lack of observability of the motor phase current in these boundary conditions reduces the capacity of the dynamic control of the motor, and prevents proper reconstruction of the motor phase currents. It would be desirable to improve the observability of motor phase currents in the boundary areas, while reducing the requirements of current sensors used to measure motor current.